Beyond the Stone Arch by Gisela Woldenga

During a visit to her grandmother’s house, Ellie notices a stone arch in the garden that she has never seen before. Curious, she steps through it and meets a boy, Orly, who has been waiting for her. He tells her that he was killed two hundred years ago, and he needs Ellie’s help to find his grave so he can go back to his village. Ellie knows that he’s a ghost, and she’s the only one who can see and talk to him. After hearing his story, she agrees to help, even though she has to go through many adventures. She’s not allowed to tell her grandmother about the details. Will Ellie succeed in convincing Grandma of her gift and giving the boy peace?